Carbohydrates are now recognized as being of major importance in many cell-cell recognition events, notably the adhesion of bacteria and viruses to mammalian cells in pathogenesis and leukocyte-endothelial cell interaction through selectins in inflammation (Varki (1993) Glycobiology 3: 97-130). Moreover, sialylated glycoconjugates that are found in bacteria (Preston et al. (1996) Crit. Rev. Microbiol. 22:139-180; Reuter et al. (1996) Biol. Chem. Hoppe-Seyler 377:325-342) are thought to mimic oligosaccharides found in mammalian glycolipids to evade the host immune response (Moran et al. (1996) FEMS Immunol. Med. Microbiol. 16:105-115). Molecular mimicry of host structures by the saccharide portion of lipopolysaccharide (LPS) is considered to be a virulence factor of various mucosal pathogens, which use this strategy to evade a host immune response (Moran et al. (1996) FEMS Immunol. Med. Microbiol. 16: 105-115; Moran et al. (1996) J. Endotoxin Res. 3: 521-531).
The oligosaccharide structures involved in these and other processes are potential therapeutic agents, but they are time consuming and expensive to make by traditional chemical means. A very promising route to production of specific oligosaccharide structures is through the use of the enzymes which make them in vivo, the glycosyltransferases. Such enzymes can be used as regio- and stereoselective catalysts for the in vitro synthesis of oligosaccharides (Ichikawa et al. (1992) Anal. Biochem. 202: 215-238).
Large scale enzymatic synthesis of oligosaccharides depends on the availability of sufficient quantities of the required glycosyltransferases. However, production of glycosyltransferases in sufficient quantities for use in preparing oligosaccharide structures has been problematic. Expression of many mammalian glycosyltransferases has been achieved involving expression in eukaryotic hosts which can involve expensive tissue culture media and only moderate yields of protein (Kleene et al. (1994) Biochem. Biophys. Res. Commun. 201: 160-167; Williams et al. (1995) Glycoconjugate J. 12: 755-761). Expression in E. coli has been achieved for mammalian glycosyltransferases, but these attempts have produced mainly insoluble forms of the enzyme from which it has been difficult to recover active enzyme in large amounts (Aoki et al. (1990) EMBO. J. 9:3171-3178; Nishiu et al. (1995) Biosci. Biotech. Biochem. 59 (9): 1750-1752). Furthermore, because of the biological activity of their products, mammalian sialyltransferases generally act in specific tissues, cell compartments and/or developmental stages to create precise glycan structures. Identification of glycosyltransferases that can be used in enzymatic synthesis of commercially valuable oligosaccharides and that can be produced in large quantities would thus be useful in the development of these technologies. The present invention fulfills this and other needs.